Foods and other packaged products are frequently sealed in packaging films. In high-quality packaging, use is made of films which are coated with a PVDC or acrylate lacquer in order to protect the print and to increase the gloss. A particular form of packaging of this type is “roll-wrap” packaging, in which flat circular products, such as, for example, cookies, are introduced in stacks in a single operation into a film tube which has a longitudinal seal seam on the enveloping surface and which has a diameter which matches very precisely. The longitudinal seam seal here can be sealed either with the inside against the inside of the packaging (fin seal) or with the inside against the outside (lap seal).
The length of the tube is such that its two ends are subsequently folded over the center of the circular end faces from four to twelve times and heat-sealed overlapping in this way. Sealing is effected here both inside against inside and inside against outside in order to achieve a substantially tightly sealed pack.
In a particular embodiment, the fin seal can be effected in such a way that one edge projects slightly over the other edge of the sealed enveloping surface, the fin seal is folded over at the sides, and the projecting part is sealed with the inside of the tube material against its outside.
The packaging material from which roll-wrap packaging of this type is shaped must be designed in such a way that sealing can be effected both inside against inside and also inside against outside of the film, with sealing advantageously taking place even at low sealing pressures in cases in which fragile products, such as cookies, are to be packed. In addition to roll-wrap packaging, numerous other packaging forms are known in which similar requirements are made.
Prior-art packaging which meets these requirements includes BOPP films which are provided with a coating of PVDC or acrylic lacquers over the print and on the opposite side, i.e. on both sides. Depending on the system, coatings of this type require the use of solvents, which are partly released into the environment during drying or are retained by means of technical measures and have to be worked up, or require a high application rate and thus high usage of materials. This is associated with costs for the materials employed and for the provision and operation of the application systems and the corresponding ancillary equipment.
British Application GB 2 223 446 describes a BOPP film which consists of at least two layers, with the comparatively thinner layer consisting of a blend of a material which has low heat seal strength to PVDC and a material which consists of a copolymer of an alkene and an unsaturated monobasic acid or an ester thereof. In preferred embodiments, suitable materials having low heat seal strength to PVDC are high- and low-density polyethylenes, and suitable copolymers are those of ethylene with acrylic acid esters, where, in particularly preferred embodiments, these copolymers can comprise unsaturated dibasic acids or anhydrides thereof, such as, for example, maleic anhydride, as further monomers. Corresponding copolymers and terpolymers have been described in EP 0 065 898.
On repetition of British Application GB 2 223 446, it was observed that the process described therein results, on use of the formulations indicated therein, in deposits on the heating and stretching rolls of the longitudinal stretching unit of a sequential BOPP machine to a large extent which is unacceptable for industrial practice. Variations within the limits of the disclosed teaching brought no advantage or only a slight advantage with respect to the amount and speed of the roll coating built up, or the sealing properties were adversely affected.
The object of the present invention was therefore to provide a biaxially oriented polyolefin film which is composed of readily available and inexpensive components and is distinguished by the fact that on the one hand it seals, by means of a top layer, to coatings or lacquers based on PVDC or acrylates, and on the other hand can be produced without the formation of deposits on the rolls of the longitudinal stretching unit. In addition, the usual service properties and optical properties of the film should not be adversely affected.